The present invention relates to a dry, freely flowing powder mix comprising a phase change material.
Phase change materials may be repeatedly converted between solid and liquid phases and utilize their latent heats of fusion to absorb, store and release energy to heat or cool during such phase conversions. Phase change materials may also be converted from a liquid to a gas and utilize their latent heats of vaporization and condensation to absorb and release energy in the form of heat for purposes of temperature control.
In phase change materials, the amount of energy absorbed upon melting or vaporizing, or released upon freezing and condensation is much greater than the amount of energy absorbed or released upon increasing or decreasing the temperature of the material over an increment of 1.degree. C.
Upon melting, freezing, vaporizing and condensing, per unit weight, a phase change material (PCM) absorbs or releases substantially more energy than a sensible heat storage material that is heated or cooled in generally the same temperature range. In contrast to a sensible heat storage material that absorbs and releases energy essentially uniformly over a broad temperature range, a phase change material absorbs and releases a large quantity of energy in the vicinity of its melting/freezing point, and an even greater quantity of energy in the vicinity of its boiling and condensation point.
Typical phase change materials of the solid-to-liquid type can store and release 40 to 80 calories per gram of phase change material. The liquid-to-gas change of state for phase change materials is generally six times as energetic as the solid-to-liquid transition (e.g., ice to water, 80 cal/g; water to steam, &gt;500 cal/g). However, the much more energetic liquid-to-gas transition has been little used for conventional temperature control purposes because of the large volume and pressure changes that normally accompany it, which changes can be especially problematic in closed environments.
Phase change materials capable of storing and releasing thermal energy have found many applications in building structures, road base materials, beverage and food containers, medical wraps, and textile applications such as garments. One of the basic problems, however, in the use of solid-to-liquid PCM's for control of temperature, is containment. That is, for heat transfer efficiency as well as safety purposes, it is undesirable to have a thick block or agglomeration of solid phase PCM below the PCM melting point. Similarly, when above the melting point, PCM in liquid phase can be problematic. For instance, building panels containing liquid phase PCM have proven deficient. In one such PCM-containing panel, carpenters reported that a liquid phase PCM leaked out of the panel when nails were driven through it. Additionally, the volume changes that accompany melting and freezing can cause problems in breaking the containing vessel unless adequate provisions are made to accommodate the volume changes. Most phase change materials expand .about.10% in melting. Water however expands .about.10% in freezing.
In those situations in which medical hot or cold packs containing PCMs are used, a solid phase agglomerate of PCM below its melting point renders the structure unwieldy and incapable of conforming about the required body part to achieve the desired heating or cooling function.
Accordingly applicant has developed a series of PCM containment systems. These are represented by U.S. Pat. Nos. 4,617,332, 4,711,813, 4,797,160, 4,908,166, and 5,053,446, all assigned to the same assignee as the present invention. However, none of the containment means disclosed in those patents involve silica. Still, the broad idea of using silica as a suspension medium for PCMs in building blocks is not new. For instance, see U.S. Pat. No. 4,259,401 (Charoudi et al) wherein this concept is disclosed at column 21, line 60 et seq. Also, Johnson et al in U.S. Pat. No. 4,237,023 discloses incorporating fumed silicon dioxide with inorganic phase-change salts which are capable of forming salt hydrates in the presence of water and Chang in U.S. Pat. No. 4,292,189 discloses a phase change energy storage system based on a combination of two inorganic salts together with nucleating and thickening agents including silicas. Finally, Allen, U.S. Pat. No. 4,008,170 describes a powdered product prepared by the vapor phase hydrolysis of a silicon compound reacted with liquid water. However, the dry water in Allen is not used as a phase change material for the storing and releasing of thermal energy.
The prior art does not suggest utilization of the combination of the preferred silica having the recited particle size and the PCM/silica weight ratios herein required in order to result in a dry, conformable, powder-like, PCM containing composition that may be useful in widespread environments.